Measuring signal to noise ratio of a WDM optical signal

ABSTRACT

A WDM optical signal is transmitted through a tunable optical filter and is polarization-nulled to find optical signal to noise ratio of individual WDM channels. The polarization nulling can be performed using a heuristic multipoint extrema search method, such as Nelder-Mead method. A plurality of checkpoints can be included in the search to verify the progress and to improve the overall robustness of a real-time polarization nulling.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority from U.S. Patent Application No.61/733,338 filed Dec. 4, 2012, which is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to optical networking equipment, and inparticular to devices and methods for measuring optical signal to noiseratio of a wavelength division multiplexed optical signal.

BACKGROUND OF THE INVENTION

In a wavelength division multiplexing (WDM) optical transmission system,optical signals at a plurality of wavelengths are encoded with digitalstreams of information. These encoded optical signals, or “wavelengthchannels”, are combined together and transmitted through a series ofspans of optical fiber in a WDM fiberoptic network. At a receiver end ofa transmission link, the wavelength channels can be separated, wherebyeach wavelength channel is individually detected by an optical receiver.

While propagating through an optical fiber, light gets attenuated viaabsorption and scattering. Yet some minimal level of optical power isrequired at the receiver end to decode information that has been encodedin a wavelength channel at the transmitter end. To boost optical signalspropagating in an optical fiber, optical amplifiers are deployed atmultiple locations, known as nodes, throughout the transmission link.Optical amplifiers extend the maximum possible length of the link, insome instances, from a few hundred kilometers to several thousandkilometers, by amplifying optical signals to power levels close to theoriginal levels of optical power at the transmitter end.

Even when amplified to original power levels, WDM optical signals cannotbe restored to their original condition, because optical amplifiers addin-band noise to the signal. The optical noise effectively limits amaximum length of the transmission link, and therefore needs to betightly controlled and measured.

One straightforward method to measure in-band signal-to-noise ratio(SNR) is to convert optical signal to electrical signal and to measurethe SNR by demodulating the electrical signal. However, thestraightforward SNR measurement requires costly electronic equipment.Thus, measuring SNR in optical domain (so-called “optical SNR” or OSNR)is highly desirable.

OSNR can be evaluated by scanning the spectrum of the WDM opticalsignal, detecting wavelength channels which reveal themselves asspectral peaks in the WDM optical signal, and evaluating ratio of peaksto valleys in the spectrum. This method, however, is inherentlyimprecise, because it assumes that optical noise in the valleys betweenthe wavelength channels is the same as in the in-band optical noise. Itis not uncommon that in-band noise is actually much higher thanout-of-band noise.

A more precise method relies on a difference between polarizationproperties of wavelength channels and optical noise. The WDM signallight is generated by laser diodes, which emit polarized light. Uponoptical amplification, the light polarization is generally preserved. Asa result, a degree of polarization of the wavelength channels is high(when polarization mode dispersion (PMD) is low enough). On the otherhand, optical noise remains unpolarized. This is because optical noisein optical fiber amplifiers originates from randomly polarizedspontaneous emission of light in optically inversed gain medium, whichis amplified by the same gain medium that emitted it. A high degree ofpolarization of the useful optical signal and lack of polarization inthe optical noise allows one to suppress wavelength channels one by oneusing polarization nulling methods, and directly measure the remainingoptical noise. The measurement is repeated for each wavelength channelof interest.

Chung et al. in US Patent Application Publication 2004/0114923 disclosean OSNR monitoring system including a polarization controller coupled toa linear polarizer and a tunable optical bandpass filter. The tunableoptical bandpass filter is tuned to a wavelength channel of interest.Since the polarization state of the wavelength channels is not known,the polarization controller scans the polarization within apredetermined range, and a minimum value is searched for. When thepolarization direction of the optical signal at the output of thepolarization controller is orthogonal to the polarization transmissiondirection of the polarizer, the transmitted optical power is at minimum,being equal to one half of the optical noise power. Once the opticalnoise power is known, the OSNR can be calculated.

Yao in U.S. Pat. Nos. 7,218,436; 7,391,977; and 8,000,610 discloses asystem for measuring OSNR by either scrambling polarization of WDMoptical signal, or by systematically varying through all possible statesof polarization, and detecting maximum and minimum optical power levelsat a photodetector disposed downstream of an optical polarizer. Thesystem of Yao also includes a tunable optical filter for selectingindividual wavelength channels of the WDM optical signal.

Detrimentally, the OSNR measuring systems of Chung and Yao rely onscanning a polarization controller through all polarization states tofind a particular setting of the polarization controller, at which theoptical signal from a particular wavelength channel is suppressed. Dueto a great multitude of possible polarization states of a polarizationcontroller, such scanning can take an impractically long time.Polarization scrambling, that is, quickly and randomly changingpolarization of the WDM optical signal, can be used in an attempt toshorten the scanning time at each wavelength. However, polarizationscrambling does not guarantee that the required polarization state isalways achieved, thus reducing fidelity of OSNR measurements.

Chung et al. in U.S. Pat. No. 7,257,324 disclose an OSNR monitoringapparatus including a polarization controller coupled to apolarization-selective optical delay line, for imparting a controllableamount of a differential group delay (DGD) to the modulated opticalsignal. A fast photodetector is coupled to the polarization-selectiveoptical delay line for measuring DC and AC components of the modulatedoptical signal. At a certain pre-defined amount of DGD imparted to theoptical signal, the DC component becomes proportional to a magnitude ofthe wavelength channel signal, while the AC component is proportional tothe optical noise. Thus, by measuring ratio of DC electrical signal toAC electrical signal at the photodetector output, OSNR can be estimated.

Detrimentally, the apparatus of Chung et al. in U.S. Pat. No. 7,257,324requires rather complex electronics for processing high-frequencyelectrical signals. Furthermore, the optimal delay has to be found inadvance before proper calculations can be carried out, the signal has tobe stable in time, and non-linear effects must not degrade the spectralcharacteristic of the signal to be measured.

SUMMARY OF THE INVENTION

The inventor has discovered that a WDM optical signal transmittedthrough a tunable optical filter can be polarization-nulled in real timeduring a scan of the optical spectrum of the WDM optical signal by thetunable optical filter. To track the constantly changing state ofpolarization of the transmitted optical signal, the polarization nullingis preferably performed using a heuristic multipoint extrema search,such as Nelder-Mead search. A plurality of checkpoints can be includedin the search to verify the progress and to improve the overallrobustness of the real-time polarization nulling. When the polarizationnulling time is much smaller than the spectrum scanning time, forexample one thousand to one hundred thousand times smaller, the OSNR ofevery WDM channel can be measured in a single sweep of the opticalspectrum.

In accordance with the invention, there is provided a device formeasuring OSNR of a WDM optical signal including a plurality ofwavelength channels, the device comprising:

an input port;

an optical train coupled to the input port and including seriallyconnected:

a tunable optical filter for selecting a wavelength channel of theplurality of wavelength channels,

a polarization controller for adjusting a polarization state of theselected wavelength channel,

a polarization selector disposed in the optical train downstream of thepolarization controller, for selecting a polarization state of theselected wavelength channel, and

a photodetector disposed in the optical train downstream of the tunableoptical filter, the polarization controller, and the polarizationselector, for detecting a first optical power level of the selectedwavelength channel and in the selected polarization state; and

a control unit coupled to the tunable optical filter, the polarizationcontroller, and the photodetector, and configured to tune the opticalfilter to the selected wavelength channel; adjust the polarizationcontroller to reach a target control point, at which the first opticalpower level is minimized or reduced; and compute the OSNR of theselected wavelength channel from a polarization extinction defined as aratio of a second optical power level upstream the polarization selectorto the first optical power level;

wherein the control unit includes a computer processor and anon-transitory memory configured for causing the computer processor toperform a heuristic iterative search of the target control point byperforming a plurality of iterations including a first iteration and asecond iteration after the first, wherein a plurality of control pointsof the second iteration are selected based on optical power levelsdetected by the photodetector at a plurality of control points of thefirst iteration.

In one embodiment, the control unit is configured to continuously scanthe tunable optical filter, while continuously adjusting thepolarization controller to reach the target control point for eachwavelength channel. Preferably, the heuristic iterative search includesa Nelder-Mead search, wherein the plurality of control points of thefirst and second iterations form first and second Nelder-Mead simplexes,respectively, in a parameter space of the polarization controller.

In accordance with the invention, there is further provided a method formeasuring OSNR of a WDM optical signal including a plurality ofwavelength channels, the method comprising:

(a) coupling the WDM optical signal to an optical train includingserially connected:

a tunable optical filter for selecting a wavelength channel of theplurality of wavelength channels,

a polarization controller for adjusting a polarization state of theselected wavelength channel,

a polarization selector disposed in the optical train downstream of thepolarization controller, for selecting a polarization state of theselected wavelength channel, and

a photodetector disposed in the optical train downstream of the tunableoptical filter, the polarization controller, and the polarizationselector, for detecting a first optical power level of the selectedwavelength channel and in the selected polarization state;

(b) tuning the optical filter to the selected wavelength channel;

(c) adjusting the polarization controller to reach a target controlpoint at which the first optical power level is minimized or reduced;and

(d) computing the OSNR of the selected wavelength channel from apolarization extinction defined as a ratio of a second optical powerlevel upstream the polarization selector to the first optical powerlevel;

wherein step (c) includes using a control unit to automatically performa heuristic iterative search of the target control point, by performinga plurality of iterations including a first iteration and a seconditeration after the first, wherein a plurality of control points of thesecond iteration are selected based on optical power levels detected bythe photodetector at a plurality of control points of the firstiteration.

In one embodiment, step (b) includes continuously scanning the tunableoptical filter, and step (c) includes continuously adjusting thepolarization controller to reach the target control point for eachwavelength channel. Preferably, the heuristic iterative search of step(c) comprises a Nelder-Mead search, wherein the plurality of controlpoints of the first and second iterations form Nelder-Mead simplexes ina parameter space of the polarization controller.

In accordance with another aspect of the invention, there is furtherprovided a device for measuring OSNR of a WDM optical signal including aplurality of wavelength channels, the device comprising:

an input port;

a tunable optical filter coupled to the input port, for selecting awavelength channel of the plurality of wavelength channels;

a polarization controller coupled to the tunable optical filter, foradjusting a polarization state of the selected wavelength channel;

a polarization selector coupled to the polarization controller, forselecting a polarization state of the selected wavelength channel;

a photodetector assembly coupled to the polarization selector, fordetecting first and second optical power levels of the selectedwavelength channel upstream and downstream of the polarization selector,respectively; and

a control unit coupled to the tunable optical filter, the polarizationcontroller, and the photodetector, and configured to tune the opticalfilter to the selected wavelength channel; adjust the polarizationcontroller to reach a target control point at which the first opticalpower level is reduced or minimized; and compute the OSNR of theselected wavelength channel from a polarization extinction defined as aratio of the second optical power level to the first;

wherein the control unit includes a computer processor and anon-transitory memory configured for scanning the tunable optical filteracross the wavelength channels of the WDM optical signal, while causingthe computer processor to continuously perform an iterative search ofthe target control point for evaluating the OSNR of each one of thewavelength channels.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings, in which:

FIG. 1 is a block diagram of a device for measuring OSNR according tothe invention;

FIG. 2 is a flow chart of a general method of measuring OSNR using thedevice of FIG. 1;

FIG. 3A is a schematic view of a three-stage polarization controller;

FIG. 3B is a view of a three-dimensional parameter space of thepolarization controller of FIG. 3A;

FIG. 3C is a view of Poincare sphere illustrating operation of thepolarization controller of FIG. 3A;

FIGS. 4A and 4B are consecutive sections of a flow chart of an exemplarymethod of measuring OSNR with the device of FIG. 1, using a Nelder-Meadsearch;

FIGS. 5A to 5D are three dimensional views of a parameter space of thepolarization controller of FIG. 1, showing evolution of simplexes of theNelder-Mead search; and

FIG. 6 is a spectral plot of first and second optical power levelsdetected by the device of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art.

Referring to FIG. 1, a device 100 for measuring OSNR of a WDM opticalsignal 102 including a plurality of wavelength channels 104 is shown.The device 100 includes an input port 106, a tunable optical filter 108for selecting a wavelength channel 105 of the plurality of wavelengthchannels 104, a polarization controller 110 for adjusting a polarizationstate of the selected wavelength channel 105, a polarization beamsplitter (PBS) 112 coupled to the polarization controller 110, forselecting a polarization state 115 of the selected wavelength channel105, and a first photodetector 121 coupled to the PBS 112, for detectinga first optical power level P₁ of the selected wavelength channel 105 inthe selected polarization state 115. An optional optical switch 109 canbe coupled between the tunable filter 108 and the polarizationcontroller 110 to allow the selected wavelength channel 105 bypass thepolarization controller 110 and the PBS 112, to measure a second opticalpower level P₂ upstream of the PBS 112. Alternatively, a secondphotodetector 122 can be coupled to the PBS 112 to detect a third powerlevel P₃ of the selected wavelength channel 105 in a polarization state116 orthogonal to the selected polarization state 115. A control unit118 is coupled to the tunable optical filter 108, the optional opticalswitch 109, the polarization controller 110, and the photodetector 121.The control unit 118 includes a computer processor 119 and anon-transitory memory 120.

In operation, the control unit 118 sends a filter control signal 151causing the tunable optical filter 108 to select the wavelength channel105, while suppressing all other wavelength channels 104 of the WDMoptical signal 102. The selected wavelength channel 105 passes throughthe optical switch 109, which forwards it to the polarization controller110. The polarization controller 110 transforms an input polarizationstate of the selected wavelength channel 105 into another state ofpolarization in dependence on a polarization control signal 152 from thecontrol unit 118.

The control unit 118 is configured to adjust the polarization controller110 by sending the polarization control signal 152 to reach a targetcontrol point, at which the first optical power level P₁ is minimized orreduced. The control unit 118 can be configured to send a switch controlsignal 153 causing the optical switch 109 to direct the wavelengthchannel 105 upstream of the PBS 112 and the polarization controller 110to the first photodetector 121, as shown with a dashed line 154, tomeasure the second optical power level P₂. Then, the control unit 118computes the OSNR of the selected wavelength channel 105 from apolarization extinction PE=P₂/P₁ using formulas and relationships knownto a person of skill in the art. Neglecting optical losses, one canassume that P₂=P₁+P₃. Therefore, one can measure P₃ using the secondphotodetector 122 instead of measuring P₂ by switching the opticalswitch 109, and calculate the PE using an modified formulaPE=(P₁+P₃)/P₁. In this embodiment, the control unit 118 receives a thirdoptical power level signal 155 from the second photodetector 122representing the third optical power level P₃.

According to the invention, the non-transitory memory 120 containsinstructions for the computer processor 119 to cause the computerprocessor 119 to perform a heuristic iterative search of the targetcontrol point. Heuristic searches are preferable over gradient-basedsearches, because the latter can be trapped in local minima and/ormisguided by measurement noise. A heuristic search performs a pluralityof iterations based on previously measured optical power levels atvarious heuristically selected “control points” of the polarizationcontroller 110. By way of a clarifying example, the search can include afirst iteration and a second iteration after the first. A plurality ofcontrol points of the second iteration are heuristically selected basedon optical power levels detected by the first photodetector 121 at aplurality of control points of the first iteration. Specific examples ofheuristic searches will be provided further below.

A generic iterative search method 200 of the target control point isillustrated in FIG. 2. In a step 202 of the method 200, initial controlpoints are selected, and the wavelength scan by the tunable opticalfilter 108 is initiated. During the first iteration, in a step 204, thefirst P₁ and second P₂ optical power levels are measured for eachinitial control point, and the polarization extinction PE is optionallycomputed for each initial point. In a step 206, a check is performedwhether maximum PE or a minimum first optical power level P₁ is found.If not, the set points are adjusted in a step 208 according to aspecific search method used. Then, during the second iteration, thefirst P₁ and second P₂ optical power levels are re-measured in the step204 for each modified control point. The process repeats until the firstoptical power level P₁ is minimized or at least lessened to apre-defined level, thus maximizing the polarization extinction PE. Thesecond optical power level P₂ can be measured only once, because theoptical power of the selected wavelength channel 105 usually does notchange appreciably during one cycle of the search 200. Once thepolarization extinction PE is maximized, the OSNR is computed in a step210.

Referring to FIG. 3A, the polarization controller 110 includes at leasttwo independent optical phase shifters, which may be controlled byvoltage, current, or any other suitable means. For fast operation, thepolarization controller 110 preferably includes three Pockels cellstages 301, 302, and 303 operated as voltage-variable optical phaseshifter.

In operation, the selected wavelength channel 105 propagates through thePockels cell stages 301 to 303 in sequence. The Pockels cell stages 301to 303 are rotated about an optical axis 305 at various angles, toensure a complete coverage of the Poincaré-sphere while transforming aninput state of polarization (SOP) to a desired output SOP. The rotationof the Pockels cell stages 301 to 303 is not shown in FIG. 3A forsimplicity. Referring to FIG. 3B, voltages V₁, V₂, and V₃ applied to thePockels cell stages 301, 302, and 303, respectively, form athree-dimensional parameter space 304. The target control pointcorresponding to the minimum first optical power level P₁ is sought inthe parameter space 304 of the polarization controller 110. In FIG. 3B,the target control point is shown symbolically at 330.

Turning to FIG. 3C with further reference to FIGS. 1, 2, and FIG. 3B, aPoincare sphere 300 illustrates operation of the polarization controller110. Principal points S1, S2, and S3 denote left circular, verticallinear, and 45 degrees linear polarizations, respectively, correspondingto a coordinate-system of the representation of Stokes-vectors. Applyingthe first voltage V₁ to the first stage 301 causes an initialpolarization state 311 to travel on a circular trajectory 312 on thePoincare sphere 300. Similarly, applying the second V₂ and the third V₃voltages causes the initial polarization state 311 to travel ondifferent trajectories, not shown, at different angles on the Poincaresphere 300. Together, the three voltages V₁ to V₃ allow transformationof any initial polarization state into any other polarization state,preferably vertical or horizontal linear polarization for use with thePBS 112 of FIG. 1, although it is not necessary, because other typepolarization “selectors” can be used in place of the PBS 12. Herein, theterm “polarization selector” is used to denote a device that selects oneof two orthogonal polarizations to send to the first photodetector 121,and to block or redirect the second, orthogonal polarization to thesecond photodetector 122. The polarization may be, for example, linearor circular. Preferably, the control unit 118 is configured, e.g. viasoftware or firmware stored on the non-transitory memory 120, tocontinuously scan the tunable optical filter 108, while continuouslyadjusting the polarization controller 110 to reach the target controlpoint 330 in the parameter space 304 for each wavelength channel 105.

To be able to reach the target control point 330 for each wavelengthchannel 104 of the WDM optical signal 102 as the tunable optical filter108 is scanned, the heuristic iterative search 200 should be able toperform at least the first and second iterations for each wavelengthchannel of the WDM optical signal 102 during a single scan of thetunable optical filter 108. As an illustrative example, when the tunableoptical filter 108 is an optical spectrum analyzer scanning the entirespectrum of the WDM optical signal 102 in 40 seconds, the response timeof the polarization controller 110 should be one millisecond or faster,to allow the polarization nulling to be 40 milliseconds or faster, thatis, to allow at least 40 measurements for one iterative search of thetarget control point 330 in the parameter space 304, thus allowing thepolarization nulling time to be 1000 times faster than a time of asingle wavelength scan. Preferably, the response time of thepolarization controller 110 is 10 microsecond or faster, to allow thepolarization nulling to be 400 microseconds or faster, thus allowing thepolarization nulling time to be 100,000 times faster than a time of asingle wavelength scan.

A specific, non-limiting example of a heuristic iterative search of thetarget control point 330 according to the invention will now bepresented. Referring to FIGS. 4A, 4B, and FIGS. 5A to 5D with furtherreference to FIG. 3B, a search method 400 (FIGS. 4A, 4B) uses aNelder-Mead search 450. The method 400 includes an initialization step402, in which a plurality of candidate starting control points for theNelder-Mead search 450 are generated. To generate the candidate startingpoints, the cubic-shaped control space 304 can be broken into eighthalf-size cubic segments 304A (only one is shown in FIG. 3B forclarity), and taking eight candidate control points x₁ to x₈ to be atgeometrical centers of the eight half-size cubes 304A. The points x₁ tox₈ are shown in FIGS. 5A and 5B. In a sorting step 404 (FIG. 4A), thepolarization extinction PE is measured for each of the candidate controlpoints x₁ to x₈. Then, the eight candidate control points x₁ to x₈ aresorted in the order of decreasing PE, and four highest-PE controlpoints, e.g. x₁, x₂, x₄, and x₆, are selected to be corners of aNelder-Mead starting simplex tetrahedron 501 (FIG. 5A). In a step 406, adistance is calculated between each of the selected control points x₁,x₂, x₄, and x₆ to their geometrical center. If the distance is smallerthan a pre-defined threshold distance (a decision block 410), then, in astep 408 the control points are varied. In another embodiment, thedistance between the selected points themselves is checked. In bothcases, the pre-defined threshold distance is selected so that at leastfor the first iteration of the method 400, the control points do notneed to be varied. The control points can be varied at later iterations,when the Nelder-Mead simplex becomes sufficiently small in size. Thechecking step 410 ensures a good set of starting points, increasing thechances of quick conversion of the Nelder-Mead search 450. In oneembodiment, the pre-defined minimum distance is not a constant butdepends upon a PE obtained during a previous iteration. The higher thePE of the previous iteration, the smaller the pre-defined minimumdistance.

If the distance is larger than the threshold distance, then theNelder-Mead search 450 begins. Although a generic Nelder-Mead search isknown, steps 412 to 434 of the Nelder-Mead search 450 will be brieflyconsidered. In a step 412, the “worst” point, that is, the point withthe lowest PE, is reflected through a geometrical center of the opposingtriangle. For example, the worst point P₄ is reflected through atriangle formed by the remaining points x₁, x₂, and x₆, as shown with aline 510 in FIG. 5A, and a measurement is taken at the reflected pointx_(r), corresponding to the point R₁ in FIG. 5A. Then, a check isperformed whether PE at the reflected point x_(r) is better (higher)than at the current “best” (i.e. the highest PE) point x_(b). If not,then in a step 414, a check is performed whether PE at the reflectedpoint x_(r) is higher than at the current “second worst” (i.e. thesecond lowest PE) point x_(z). If not, then in a step 416, a check isperformed whether PE at the reflected point x_(r) is higher than at thecurrent “worst” (i.e. the lowest PE) point x_(s). If not, then in a step422, the reflected point x_(r) becomes the worst point x_(s).

If PE(x_(r))>PE(x_(b)), then in a step 418, an “extended” point x_(e) ofthe Nelder-Mead search 450 is calculated, and PE at that point PE(x_(e))is measured. Then, in a step 424, a check is performed whetherPE(x_(e))>PE(x_(r)). If not, x_(r) becomes the worst point in a step426. If yes, x_(e) becomes the worst point in a step 430.

After the step 422 of assigning x_(r) as the worst point x_(s), or ifPE(x_(r))>PE(x_(s)) in the step 416, a contraction point x_(k) iscalculated and PE at that point PE(x_(k)) is measured in a step 420.Then, in a step 428, a check is performed if PE(x_(k)) is larger thanPE(x_(s)) of the current worst point x_(s). If no, then in a step 432,all points x_(i) (x₁, x₂, x₃, etc.) are shrunk (middle of vectorpointing from x_(i) to x_(b)) in the parameter space, and the new valuesare used for further calculations. If yes, then in a step 434, thecontraction point x_(k) becomes the worst point x_(s).

Steps 412 to 434 of the Nelder-Mead search 450 are repeated insubsequent iterations, resulting in a subsequent contraction of theNelder-Mead simplex 501. For example, referring specifically to FIG. 5B,a second reflection point R₂ can be calculated by reflecting the pointx₆ of a shrunk simplex tetrahedron 502 through a geometrical center of atriangle formed by the remaining three vertices x₁, R₁, and x₂.Referring specifically to FIG. 5C, a third reflection point R₃ can becalculated by reflecting the point x₁ of a further shrunk simplextetrahedron 503 through a geometrical center of a triangle formed by theremaining three vertices R₁, R₂, and x₂. As a result, a smaller simplextetrahedron 504 is formed, containing therein the target control point330. The process can be repeated to shrink the smaller simplextetrahedron 504 even further, thereby finding the target control point330 with a better precision.

Referring specifically to FIG. 4B, a number of checks is performed inthe method 400 to make sure that the Nelder-Mead search 450 does not getstuck on some secondary maximum or otherwise lacks progress in findingthe target point 330. Specifically, in a step 436, a check is performedif the target control point 330 is reached during a pre-defined numberof iterations of the Nelder-Mead search 450. For example, a check can beperformed whether the lowest first optical power level P₁ is obtainedwithin 25 latest iterations. If not, then in a step 440 the firstoptical power level P₁ is re-measured at the current best control pointx_(b). Then, in a step 438, a check is performed if the PE at the targetcontrol point 330 is higher than a pre-defined threshold value. Thecheck 436 is performed periodically, after a pre-defined number ofiterations. For example, the check 436 can be performed every 25iterations whether the current best PE is larger than 20 dB. If yes,then in a step 444, the current best PE is re-evaluated by firstmeasuring P₁(x_(b)) and then calculating the PE=P₁(x_(b))/P₂(x_(b)).

In a step 442, a check is performed whether a difference between PE at acurrent best point x_(b) and at a reflection point x_(e) of the currentbest point x_(b) is larger than a pre-defined threshold, e.g. 10 dB. Thecheck 442 is performed after a pre-defined number of iterations, e.g.160 iterations. The check 442 is performed to make sure that thecurrently found point provides PE values that are sufficiently higherthan neighboring values. If yes, then in a step 446 the current bestpoint x_(b) is used as the target control point 330 for computing theOSNR, and a new control loop is started, the current best point x_(b)being transferred to the new loop. If not, then the search is consideredto have failed, and a new set of starting points is generated in a step448. The new set of starting points is preferably generated using thefollowing equations (1) to (8):

$\begin{matrix}{x_{1} = \begin{pmatrix}{V\; 1_{b}} \\{V\; 2_{b}} \\{V\; 3_{b}}\end{pmatrix}} & (1) \\{x_{2} = \begin{pmatrix}{{V\; 1_{b}} + m} \\{V\; 2_{b}} \\{V\; 3_{b}}\end{pmatrix}} & (2) \\{x_{3} = \begin{pmatrix}{{V\; 1_{b}} - m} \\{V\; 2_{b}} \\{V\; 3_{b}}\end{pmatrix}} & (3) \\{x_{4} = \begin{pmatrix}{V\; 1_{b}} \\{{V\; 2_{b}} + m} \\{V\; 3_{b}}\end{pmatrix}} & (4) \\{x_{5} = \begin{pmatrix}{V\; 1_{b}} \\{{V\; 2_{b}} - m} \\{V\; 3_{b}}\end{pmatrix}} & (5) \\{x_{6} = \begin{pmatrix}{V\; 1_{b}} \\{V\; 2_{b}} \\{{V\; 3_{b}} + m}\end{pmatrix}} & (6) \\{{x_{7} = \begin{pmatrix}{V\; 1_{b}} \\{V\; 2_{b}} \\{{V\; 3_{b}} - m}\end{pmatrix}},{and}} & (7) \\{x_{8} = \begin{pmatrix}{V\; 1_{2b}} \\{V\; 2_{2b}} \\{V\; 3_{2b}}\end{pmatrix}} & (8)\end{matrix}$

wherein V1_(b), V2_(b), and V3_(b) are coordinates (that is, voltages,see FIG. 3A) of the current best point; V1_(2b), V2_(2b), and V3_(2b)are coordinates of a current second best point; and m is a parameter.Preferably, the parameter m is selected dynamically, depending on the PEat the best point x_(b). The higher the PE at the best point x_(b), thesmaller the selected parameter m.

After the new set of starting points x₁ to x₈ is generated in the step448, the quick-sorting step 404 (FIG. 4A) is performed for the next,second iteration, and the whole process 400 repeats.

Turning now to FIG. 6 with further reference to FIG. 1, spectral plots601 and 602 of first P₁ and second P₂ optical power levels are shown,respectively. The first P₁ and second P₂ optical power levels have beendetected by the device 100 of FIG. 1 in two consecutive scans of thetunable optical filter 108 across the spectrum of the selectedwavelength channel 105, at two different states of the optical switch109. The polarization nulling time in this example was much shorter thanthe scan time, allowing the polarization nulling 400 to arrive at thetarget control point for each wavelength of the scanned spectrum 501,that is, in real time. During the second scan, the polarization nullingwas switched off, the optical switch 109 directing the optical signalfor measurement of the second optical power P₂. The PE values of up to24 dB were thus measured for the selected wavelength channel 105.Alternatively, if the second photodetector 122 is provided in the device100, the optical switch 109 is not required, and both optical powerlevels P₁ and P₃ can be measured during a single scan of the spectrum bythe tunable optical filter 108. The tunable filter 108 can include adiffraction grating based optical spectrum analyzer, a tunableFabry-Perot etalon filter, and the like. The polarization controller 110can include more than the minimal needed amount of stages, for “endless”polarization control. Rotatable waveplate controllers, fiber squeezingcontrollers, and other types of polarization controllers may be used.The minimal number of stages depends on the technology used.

Furthermore, elements of an optical train including the tunable opticalfilter 108, the polarization controller 110, the polarization selector112, and the first photodetector 121 can be switched, as long as thepolarization selector 112 is disposed downstream of the polarizationcontroller 110, and the first photodetector 121 is disposed in theoptical train downstream of the tunable optical filter 108, thepolarization controller 110, and the polarization selector 112. Forexample, the polarization controller can be the first element in theoptical train disposed upstream of the tunable optical filter 108.

A general method of measuring OSNR of the WDM optical signal 102 canthus include (a) coupling the WDM optical signal 102 to the opticaltrain of the device 100, (b) tuning the optical filter 108 to theselected wavelength channel 105, (c) adjusting the polarizationcontroller 110 to reach the target control point 330, and (d) computingthe OSNR of the selected wavelength channel 105 from the measured PE.Step (c) includes using the control unit 118 to automatically performthe heuristic iterative search 200 or 400 of the target control point,by performing a plurality of iterations as explained above.

Preferably, step (b) includes continuously scanning the tunable opticalfilter 108, and step (c) includes continuously adjusting thepolarization controller 110 to reach the target control point 330 foreach wavelength channel scanned. Further, preferably, the heuristiciterative search 400 comprises the Nelder-Mead search 450, in which theplurality of control points of the first and second iterations formfirst and second Nelder-Mead simplexes, respectively, in the parameterspace 304 of the polarization controller 110. As explained above withreference to FIGS. 4A and 4B, the step of adjusting polarization caninclude computing, after a pre-defined number of iterations of theNelder-Mead search, a difference between polarization extinction valuesat a current best point and at a reflection point of the current bestpoint, and using the current best point as the selected control pointfor computing the OSNR when the difference is larger than a pre-definedamount.

The hardware used to implement the various illustrative logics, logicalblocks, modules, and circuits described in connection with the aspectsdisclosed herein, e.g. the controller 118 of FIG. 1, may be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general-purpose processor may be a microprocessor, but, in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration. Alternatively, some steps or methods may be performed bycircuitry that is specific to a given function.

The foregoing description of one or more embodiments of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

What is claimed is:
 1. A device for measuring OSNR of a WDM opticalsignal including a plurality of wavelength channels, the devicecomprising: an input port; an optical train coupled to the input portand including serially connected a tunable optical filter for selectinga wavelength channel of the plurality of wavelength channels, apolarization controller for adjusting a polarization state of theselected wavelength channel, a polarization selector disposed in theoptical train downstream of the polarization controller, for selecting apolarization state of the selected wavelength channel, and aphotodetector disposed in the optical train downstream of the tunableoptical filter, the polarization controller, and the polarizationselector, for detecting a first optical power level of the selectedwavelength channel and in the selected polarization state; and a controlunit coupled to the tunable optical filter, the polarization controller,and the photodetector, and configured to tune the optical filter to theselected wavelength channel; adjust the polarization controller to reacha target control point, at which the first optical power level isminimized or reduced; and compute the OSNR of the selected wavelengthchannel from a polarization extinction defined as a ratio of a secondoptical power level upstream the polarization selector to the firstoptical power level; wherein the control unit includes a computerprocessor and a non-transitory memory configured for causing thecomputer processor to perform a heuristic iterative search of the targetcontrol point by performing a plurality of iterations including a firstiteration and a second iteration after the first, wherein a plurality ofcontrol points of the second iteration are selected based on opticalpower levels detected by the photodetector at a plurality of controlpoints of the first iteration.
 2. The device of claim 1, wherein thecontrol unit is configured to continuously scan the tunable opticalfilter, while continuously adjusting the polarization controller toreach the target control point for each wavelength channel.
 3. Thedevice of claim 2, wherein the heuristic iterative search and thepolarization controller are configured to perform at least the first andsecond iterations for each wavelength channel of the WDM optical signalduring a single scan of the tunable optical filter.
 4. The device ofclaim 1, wherein each of the plurality of control points of the seconditeration is separated from every other control point of the pluralityof control points of the second iteration by a pre-defined minimumdistance in a parameter space of the polarization controller.
 5. Thedevice of claim 1, wherein each of the plurality of control points ofthe second iteration is separated from a center of gravity of theplurality of control points of the second iteration by a pre-definedminimum distance in a parameter space of the polarization controller. 6.The device of claim 5, wherein the pre-defined minimum distance dependsupon a polarization extinction obtained during the first iteration,wherein the higher the polarization extinction, the smaller thepre-defined minimum distance.
 7. The device of claim 1, wherein thecontrol unit is configured to determine if the target control point isreached during a pre-defined number of iterations of the heuristiciterative search and, if not, to re-measure the first optical powerlevel at the target control point.
 8. The device of claim 7, wherein thecontrol unit is configured to periodically determine if the polarizationextinction at the target control point is higher than a pre-definedthreshold value, and if yes, to re-measure the first optical power levelat the target control point.
 9. The device of claim 8, wherein thepre-defined threshold value is at least 20 dB, or wherein thepre-defined number of iterations is at least 25 iterations.
 10. Thedevice of claim 1, wherein the heuristic iterative search comprises aNelder-Mead search, wherein the plurality of control points of the firstand second iterations form first and second Nelder-Mead simplexes,respectively, in a parameter space of the polarization controller. 11.The device of claim 10, wherein the control unit is further configuredto compute, after a pre-defined number of iterations of the Nelder-Meadsearch, a difference between polarization extinction values at a currentbest point and at a reflection point of the current best point, and touse the current best point as the target control point for computing theOSNR when the difference is larger than a pre-defined amount.
 12. Thedevice of claim 11, wherein the pre-defined amount is 10 dB.
 13. Thedevice of claim 11, wherein, when the difference is less than thepre-defined amount, the control unit is configured to restart theNelder-Mead search with a starting simplex having four control pointswith the highest extinction ratios among the following eight points x₁to x₈: $\begin{matrix}{{x_{1} = \begin{pmatrix}{V\; 1_{b}} \\{V\; 2_{b}} \\{V\; 3_{b}}\end{pmatrix}},} \\{{x_{2} = \begin{pmatrix}{{V\; 1_{b}} + m} \\{V\; 2_{b}} \\{V\; 3_{b}}\end{pmatrix}},} \\{{x_{3} = \begin{pmatrix}{{V\; 1_{b}} - m} \\{V\; 2_{b}} \\{V\; 3_{b}}\end{pmatrix}},} \\{{x_{4} = \begin{pmatrix}{V\; 1_{b}} \\{{V\; 2_{b}} + m} \\{V\; 3_{b}}\end{pmatrix}},} \\{{x_{5} = \begin{pmatrix}{V\; 1_{b}} \\{{V\; 2_{b}} - m} \\{V\; 3_{b}}\end{pmatrix}},} \\{{x_{6} = \begin{pmatrix}{V\; 1_{b}} \\{V\; 2_{b}} \\{{V\; 3_{b}} + m}\end{pmatrix}},} \\{{x_{7} = \begin{pmatrix}{V\; 1_{b}} \\{V\; 2_{b}} \\{{V\; 3_{b}} - m}\end{pmatrix}},{and}} \\{{x_{8} = \begin{pmatrix}{V\; 1_{2b}} \\{V\; 2_{2b}} \\{V\; 3_{2b}}\end{pmatrix}},}\end{matrix}$ wherein V1_(b), V2_(b), and V3_(b) are coordinates of thecurrent best point; V1_(2b), V2_(2b), and V3_(2b) are coordinates of acurrent second best point; and m is a parameter.
 14. The device of claim13, wherein the parameter m is selected dynamically, depending on thepolarization extinction at the best point, wherein the higher thepolarization extinction at the best point, the smaller the parameter m.15. The device of claim 1, further comprising an optical switch forcoupling the tunable optical filter directly to the photodetector, formeasuring the second optical power.
 16. A method for measuring OSNR of aWDM optical signal including a plurality of wavelength channels, themethod comprising: (a) coupling the WDM optical signal to an opticaltrain including serially connected a tunable optical filter forselecting a wavelength channel of the plurality of wavelength channels,a polarization controller for adjusting a polarization state of theselected wavelength channel, a polarization selector disposed in theoptical train downstream of the polarization controller, for selecting apolarization state of the selected wavelength channel, and aphotodetector disposed in the optical train downstream of the tunableoptical filter, the polarization controller, and the polarizationselector, for detecting a first optical power level of the selectedwavelength channel and in the selected polarization state; (b) tuningthe optical filter to the selected wavelength channel; (c) adjusting thepolarization controller to reach a target control point at which thefirst optical power level is minimized or reduced; and (d) computing theOSNR of the selected wavelength channel from a polarization extinctiondefined as a ratio of a second optical power level upstream thepolarization selector to the first optical power level; wherein step (c)includes using a control unit to automatically perform a heuristiciterative search of the target control point by performing a pluralityof iterations including a first iteration and a second iteration afterthe first, wherein a plurality of control points of the second iterationare selected based on optical power levels detected by the photodetectorat a plurality of control points of the first iteration.
 17. The methodof claim 16, wherein step (b) includes continuously scanning the tunableoptical filter, and step (c) includes continuously adjusting thepolarization controller to reach the target control point for eachwavelength channel.
 18. The method of claim 17, wherein the heuristiciterative search of step (c) comprises a Nelder-Mead search, wherein theplurality of control points of the first and second iterations formfirst and second Nelder-Mead simplexes, respectively, in a parameterspace of the polarization controller.
 19. The method of claim 18,wherein step (c) includes computing, after a pre-defined number ofiterations of the Nelder-Mead search, a difference between polarizationextinction values at a current best point and at a reflection point ofthe current best point, and using the current best point as the selectedcontrol point for computing the OSNR in step (d) when the difference islarger than a pre-defined amount.
 20. A device for measuring OSNR of aWDM optical signal including a plurality of wavelength channels, thedevice comprising: an input port; a tunable optical filter coupled tothe input port, for selecting a wavelength channel of the plurality ofwavelength channels; a polarization controller coupled to the tunableoptical filter, for adjusting a polarization state of the selectedwavelength channel; a polarization selector coupled to the polarizationcontroller, for selecting a polarization state of the selectedwavelength channel; a photodetector assembly coupled to the polarizationselector, for detecting first and second optical power levels of theselected wavelength channel upstream and downstream of the polarizationselector, respectively; and a control unit coupled to the tunableoptical filter, the polarization controller, and the photodetector, andconfigured to tune the optical filter to the selected wavelengthchannel; adjust the polarization controller to reach a target controlpoint at which the first optical power level is reduced or minimized;and compute the OSNR of the selected wavelength channel from apolarization extinction defined as a ratio of the second optical powerlevel to the first; wherein the control unit includes a computerprocessor and a non-transitory memory configured for scanning thetunable optical filter across the wavelength channels of the WDM opticalsignal, while causing the computer processor to continuously perform aniterative search of the target control point for evaluating the OSNR ofeach one of the wavelength channels.